Over the years, several methods of administering biologically-effective materials to mammals have been proposed. Many medicinal agents are available as water-soluble salts and can be included in pharmaceutical formulations relatively easily. Problems arise when the desired medicinal is either insoluble in aqueous fluids or is rapidly degraded in vivo. Alkaloids are often especially difficult to solubilize.
For example, several methods have been suggested to overcome the problems associated with administering paclitaxel, (also known as Taxol.RTM., Bristol-Myers Squibb Co. NY, N.Y.), which is insoluble in water. Currently, taxol is administrated in physical admixture with a non-aqueous vehicle, cremophor-EL. This formulation, however, has several drawbacks. Hypersensitivity reactions have been associated with the vehicle and intravenous administration of the agent with this vehicle is also slow and causes discomfort to the patient.
Several methods have been suggested to enhance the aqueous solubility of taxol. See, for example, PCT WO 93/24476, U.S. Pat. No. 5,362,831, and Nicolaou, et al. Angew. Chem. Int. Ed. Engl. (1994) 33, No. 15/16, pages 1583-1587. Preparing water-soluble prodrug versions has also been explored.
Prodrugs include chemical derivatives of a biologically-active parent compound which, upon administration, will eventually liberate the active parent compound in vivo. Use of prodrugs allows the artisan to modify the onset and/or duration of action in vivo. In addition, the use of prodrugs can modify the transportation, distribution or solubility of a drug in the body. Furthermore, prodrugs may reduce the toxicity and/or otherwise overcome difficulties encountered when administering pharmaceutical preparations.
A typical example in the preparation of prodrugs can involve conversion of alcohols or thioalcohols to either organic phosphates or esters. Remington's Pharmaceutical Sciences, 16th Ed., A. Osol, Ed. (1980), the disclosure of which is incorporated by reference herein.
Prodrugs are often biologically inert or substantially inactive forms of the parent or active compound. The rate of release of the active drug is influenced by several factors including the rate of hydrolysis of the converted ester or other functionality.
Recently, polyethylene glycol and related polyalkylene oxides have been suggested as possible adjuncts for the preparation of taxol prodrugs. See PCT WO 93/24476 supra, for example. PEG has also been conjugated to proteins, peptides and enzymes to increase aqueous solubility and circulating life in vivo as well as reduce antigenicity. See, for example, U.S. Pat. Nos. 5,298,643 and 5,321,095, both to Greenwald, et al. These latter two references disclose, inter alia, biologically-active conjugates having substantially hydrolysis-resistant bonds (linkages) between a polyalkylene oxide and the target moiety. Thus, long-lasting conjugates rather than prodrugs per se were prepared. In most situations, the average molecular weight of the polymer included in the conjugate was preferably about 5,000 daltons.
PCT WO 93/24476 discloses using an ester linkage to covalently bind taxol to water-soluble polyethylene glycols and provide a prodrug. Applicants, however, have discovered that the ester linkages described therein provide t.sub.1/2 for hydrolysis of greater than four days in aqueous environments. Thus, most of the conjugate is eliminated prior to hydrolysis being achieved in vivo. It would be preferable to provide an ester linkage which allows more rapid hydrolysis of the polymer-drug linkage in vivo so as to generate the parent drug compound more rapidly.
It has also been surprisingly found that when only one or two polymers of less than 10,000 molecular weight are conjugated to alkaloids and/or organic compounds, the resulting conjugates are rapidly eliminated in vivo. In fact, such conjugates are so rapidly cleared from the body that even if a hydrolysis-prone ester linkage is used, not enough of the parent molecule is regenerated in vivo to make the PAO-drug conjugate worthwhile as a prodrug.
Ohya, et al., J. Bioactive and Compatible Polymers Vol. 10 January, 1995, 51-66, disclose doxorubicin-PEG conjugates which are prepared by linking the two substituents via various linkages including esters. The molecular weight of the PEG used, however, is only about 5,000 at best. Thus, the true in vivo benefits would not be realized because the conjugates would be substantially excreted prior to sufficient hydrolysis of the linkage to generate the parent molecules.
Yamaoka, et al. J. Pharmaceutical Sciences, Vol. 83, No. 4, April 1994, pages 601-606, disclose that the half-life of unmodified PEG in circulation of mice after IV administration extended from 18 minutes to one day when molecular weight was increased from 6,000 to 190,000. Yamaoka, et al., however, failed to consider the effect of linking the polymer to a drug would have on the drug. Also, Yamaoka, et al. failed to consider that aqueous solutions of higher molecular weight polymers are quite viscous and difficult to dispense through the narrow-bore devices used to administer pharmaceutical preparations.
In summary, previous prodrugs based on conjugates of parent drug compounds with water soluble polymers have not been successful due to a combination of excessively slow hydrolysis of the polymer from the parent drug and excessively rapid clearance of the prodrug from the body.